Conventionally, polyethylenes with an extremely high molecular weight, so-called ultra high molecular weight polyethylenes, have been used for many use applications since they have good properties such as impact resistance. Above all, ultra high molecular weight polyethylene fibers produced by a production method involving extruding a polyethylene solution obtained by dissolving an ultra high molecular weight polyethylene in an organic solvent by an extruder, thereafter quenching the resulting solution to form a fibrous gel body, and continuously drawing the gel body while removing the organic solvent from the gel body (hereinafter, referred to as gel spinning method) have widely been known as fibers with high strength and high elastic modulus (e.g., Patent Document 1 and Patent Document 2).
It is also known that fibers with high strength and high elastic modulus can be produced by a dry spinning method involving spinning a spinning solution obtained by evenly dissolving an ultra high molecular weight polyethylene in a volatile solvent, evaporating the solvent from the spun gel thread, then cooling the gel thread by using an inert gas, and finally drawing the gel thread at a high draw ratio (e.g., Patent Document 3).
Higher strength and higher elastic modulus of the filament are known to be achieved by making mechanical properties or crystal orientation between monofilaments uniform. The mechanical properties such as strength and elastic modulus are excellent, but dimensional stability and abrasion resistance are poor, so that there are some problems in that when the filament is formed into a rope or a braid, its shape is easily deformed, or low abrasion resistance frequently causes monofilament breakage during processing, resulting in easy occurrence of fluff during product use (e.g., Patent Document 4).
As described above, polyethylene fibers (multifilaments) with high strength and high elastic modulus have widely been used in recent years. However, when polyethylene fibers with improved strength and elastic modulus are used for, for example, ropes and braids, designs with a less number of fibers for braiding or low titer are made possible, and this makes it possible to narrow diameters of ropes or braids. However, it results in a defect that abrasion resistance becomes poor.
Especially, braids made of multifilaments or monofilaments are used for many use applications such as fishing lines, nets, blind cords and ropes. As use applications of these braids have been diversified, the braids are required to have functionality corresponding to required properties of products. For example, in the case of a fishing line, various properties are required in accordance with types of fish to be fished and ways for fishing. Although fishing lines made of conventionally used ultra high molecular weight polyethylene fibers are excellent in high strength and high elastic modulus, they have a problem of being easily changeable in dimensions and physical properties because of uneven microstructure in the interior of the fibers. Accordingly, in the case of fishing lines production, there occurs the problem that not only dimensional stability is poor but also abrasion resistance, which is one of important factors as fishing lines, is poor.
In addition, when fishing lines made of ultra high molecular weight polyethylene fibers are used for a long time, the braided filaments are gradually fastened one another with the lapse of time, and the fishing lines lose flexibility that is an important factor as fishing lines, and gradually become hard. When the fishing lines become hard, dimensional change is generated, and along with the change, there occurs the problem that the physical properties change.
As a means for solving such a problem, Patent Document 5 describes a cord obtained by subjecting a braid to a heat treatment after production of the braid. The heat treatment can suppress the cord from being fluctuated in mechanical properties. However, when the cord is used as a fishing line, there occurs the problem that the braid tends to be worn and is deteriorated in throwing property as a fishing rod, due to not only the reason that a bundling property of fiber yarns constituting the braid is weak and consequently the braided fiber yarns are gradually fastened with lapse of time to change their dimensions, but also the reason that a cross-section of the fiber yarn forms a flat shape and consequently the friction between the fiber yarn and a fishing rod guide increases.
On the other hand, a braid obtained by using a twisted yarn made of various synthetic fibers or natural fibers as a core yarn and coating the core yarn with braided yarns of various fibers has conventionally been used for a blind cord to be used for lifting blinds. Since a blind cord is used for lifting blinds, it is important that the blind cord shows less dimensional change even after repeat use, and the braid is less twisted back. Further, since a blind cord is used for a long time, it is also important that the blind cord shows little change of physical properties such as expansion and contraction in relation to environmental change of temperature and humidity.
However, for large scale blinds which have been used in recent years, a blind cord is worn more severely than before by lifting such blinds. Accordingly, a conventional blind cord is hard to sufficiently exhibit functions due to low abrasion resistance and significant physical property change in the case of being used as a blind cord for large scale blinds. Accordingly, it is eagerly desired to make a blind cord available which is more excellent in performance, particularly, excellent in abrasion resistance.